Final answer:
When substrate concentration is low, it is the limiting factor for reaction velocity in enzyme-catalyzed reactions. As more substrate is added, reaction rate increases until the enzymes are saturated, after which no further increase in rate occurs.
Step-by-step explanation:
When the substrate concentration is low, the limiting factor in determining the reaction velocity is indeed the availability of the substrate itself. Since the active sites of the enzyme molecules are not all occupied due to the low substrate levels, an increase in substrate concentration will lead to a proportionate increase in the rate of the enzymatic reaction, up to a point of saturation.
If the concentration of substrate X is doubled in a scenario where it is initially low, the rate of the enzyme-catalyzed reaction will increase. However, if the enzyme concentration is kept constant and the substrate concentration keeps rising, a point would be reached where all the enzyme active sites are occupied, after which the reaction velocity cannot increase any further and becomes constant. This is known as the enzyme saturation point. The relationship between substrate concentration and reaction velocity typically forms a hyperbolic curve when graphed.
In summary, when the substrate concentration is low and other conditions are stable, it is the concentration of the substrate itself that predominantly determines the rate at which the reaction proceeds. Utilizing more substrate will continue to increase the reaction rate until the enzyme active sites reach their saturation point.